Biosensors which use a field-effect transistor (FET) have been proposed (see Patent Literatures 1 to 3). Generally, field-effect transistor-based biosensors include a field-effect transistor and a reaction field for a detection target, which is formed over a channel. The reaction field is provided with a reaction film with which the detection target is bound. The biosensor applies a gate voltage to the field-effect transistor by a gate electrode from above the reaction film, and measures a source-drain current at this time to determine the presence or absence, concentration etc. of the detection target, which has been provided on the reaction field. The FET-based biosensors exhibit very high sensitivity, and therefore there has been growing expectations for practical use.